1. Field of the Invention
This invention relates to the field of photonic-crystal fibres and photonic-crystal fibre devices.
2. Description of the Related Art
Single-mode optical fibres are widely used in diverse applications in fields including telecommunications and sensing. A typical optical fibre of the prior art (referred to hereunder as a “standard fibre”) is made entirely from solid transparent materials, such as glass. The transparent material in one part (usually the middle) of a cross-section through the fibre has a higher refractive index than transparent material in the rest of the cross-section (the “cladding”) and forms an optical “core” within which light is guided by total internal reflection.
Although light guided by total internal reflection in a fibre propagates predominantly in the high-index core, some light extends into the surrounding, lower-index cladding. That light is called the “evanescent wave”; its intensity decreases quasi-exponentially with distance from the core and is usually negligible towards the periphery of the fibre. A number of technologically important devices exploit interactions with this evanescent wave. For example, a fibre can be made that contains two cores that are close enough to each other for their evanescent waves to overlap; that allows light to couple from one core to the other, so that the fibre acts as a directional coupler. In another example of a device, one polarisation of the evanescent wave interacts preferentially with a nearby plane metal surface, so that a fibre polariser is formed.
Although special fibres like two-core fibres and D-fibres can be designed so that the evanescent wave is accessible, in standard fibres the evanescent wave at the outer surface is negligible; that is desirable in order to minimise loss and sensitivity to the environment. Local access to the evanescent wave can be obtained in such a fibre by the abrasive removal of cladding material. Typically, in such a process, the fibre is embedded in a glass block and polished so that a flat surface is produced. Polishing continues until the flat surface reaches close enough to the fibre's core for the evanescent wave at the surface to be significant. Placing the flat surfaces of two such fibres in contact can then produce a directional coupler.
In standard fibres, the evanescent wave generally extends less far into the cladding for short wavelengths than it does for longer wavelengths. The light is thus more tightly confined to the core and any evanescent interaction is weaker. Although the rate of change of the strength of the interaction with wavelength can often be varied, through control of the core size and difference in refractive index between the core and the cladding, the trend in behaviour is usually the same. Thus, if the strength of interaction in an evanescent-wave device is optimal at one wavelength, it will be too strong at longer wavelengths (perhaps causing loss) and too weak at shorter wavelengths. For example, an evanescent wave directional coupler will only yield a given beam-splitting ratio at one wavelength (or a set of isolated wavelengths). However, it is often desirable for the functional properties of an optical device to be substantially independent of wavelength.
In some spectral filtering applications, a substantial evanescent interaction is required over one wavelength range of interest but not over another. In a device based on a standard fibre, it will be the longer wavelength range for which there is a stronger evanescent interaction. However, it may sometimes be desirable for the stronger evanescent interaction to occur for the shorter wavelength range.
The extent to which light is confined to the core of a fibre is related to the fibre's so-called “V-value”, defined as                               V          =                                                    2                ⁢                πρ                            λ                        ⁢                                                            n                  co                  2                                -                                  n                  cl                  2                                                                    ,                            (        1        )            where ρ is the core radius, λ is the wavelength, and nco and ncl are the refractive indices of the core and cladding material respectively. As V increases, the evanescent wave is more tightly confined to the vicinity of the core.
An entirely new type of optical fibre has been developed recently, called the photonic-crystal fibre (PCF). A conventional PCF is made from a single transparent material such as fused silica glass. The material incorporates a regular array of air-filled holes that run along the length of the fibre, at least one of which holes is missing (see, for example, J. C. Knight et al., Optics Letters 21 (1996) 1547.). The site of a missing hole is a region of locally raised refractive index and such a region can act as a waveguiding fibre core.
Light is guided in the core by a form of total internal reflection. The fibre's cladding, incorporating low-index air holes as well as high-index glass, has a lower effective refractive index than the core, which is high-index glass only. Calculations of the effective cladding index show that the shorter the wavelength of light propagating in the fibre, the higher is the effective cladding index, and in the limit of short wavelengths, the effective cladding index approaches the index of the glass (T. A. Birks et al., Optics Letters 22 (1997) 961); that is because, at shorter wavelengths, the optical field in the cladding becomes more concentrated in the glass and avoids the holes, thus raising the effective refractive index of the cladding. However, the core index is always greater, so the core will guide light.
The refractive indices and the core radius of a standard fibre are approximately independent of wavelength and so V is inversely proportional to wavelength, resulting in the trend in behaviour described above. In a conventional PCF, the effective cladding index plays the role of ncl in equation (1); V is again an increasing function of inverse wavelength, so the evanescent wave behaves similarly to an evanescent wave in a standard fibre: it becomes more tightly confined to the vicinity of the core at lower wavelengths, thus decreasing the strength of any evanescent interaction.